Processes for pyrolyzing biomass. A catalyst is used to both pyrolyze and deoxygenate the biomass within the pyrolysis zone. A source of carbon monoxide is also passed to the pyrolysis reactor. The source of carbon monoxide may comprise a stream of gas that includes carbon monoxide, or a material capable of generating or being converted in carbon monoxide within the pyrolysis zone. The carbon monoxide may be used as a reactant for a water gas shift reaction or as a reducing agent to remove oxygen from oxygenated hydrocarbons. The catalyst preferably comprises iron (III) oxide.